I. Field of the Invention
The present invention relates generally to wireless communications systems. More specifically, the present invention relates to a method and system for reducing power consumption in a wireless communications device.
II. Description of the Related Art
Wireless communications devices (WCDs) typically operate in either an active state or a standby state. In an active state (sometimes referred to as a call state), a WCD exchanges user information on a payload traffic channel, such as a radio frequency (RF) channel used for voice and/or data calls. However, when user information is not being exchanged, a WCD operates in a standby state. In a standby state, a WCD monitors a paging channel for paging channel messages transmitted by other nodes within a wireless communications system (WCS), such as base stations, satellites, and/or other WCDs. Examples of paging channel messages include ring messages that are precursors to call initiation (i.e., active state operation), and messages that update a WCD's operational parameters.
One type of paging channel is a slotted paging channel. Slotted paging channels include a plurality of periodic time slots, wherein each WCD in a WCS is assigned to one of the plurality of time slots. A WCD monitors paging channel transmissions during its assigned time slot. Moreover, messages to a particular WCD are transmitted only during the time slot assigned to that particular WCD. Since a WCD may receive paging channel messages during its assigned time slot, it exists in an “awake” mode during this slot. During awake mode operation, components within a WCD are configured for paging channel message reception. A WCD typically begins awake mode operation before the beginning of its assigned paging channel time slot. Furthermore, a WCD may continue to operate in this awake mode after its assigned time slot if a received paging channel message requires the WCD to perform additional actions, such as receive additional transmissions.
Once there are no additional active mode actions to be performed, a WCD may operate in a “sleep” mode during the interval between successive occurrences of its assigned slot. During sleep mode operation, a WCD may conserve energy by shutting off power to some of its components. For example, a WCD may halt operation of certain components used for reception and processing that are not needed during time intervals when no information will be directed to that particular WCD. This halting typically “freezes” internal processes, such as counters and sequence generators. In addition, this halting may also power down certain other components.
In WCSs that operate according to CDMA standards, such as IS-95A and IS-95B (referred to herein as IS-95 systems), information, such as paging channel traffic, is sent between transmitting nodes (transmitters) and receiving nodes (receivers) in the form of symbol sequences. This information is interleaved, encoded, spread according to pseudonoise (PN) sequences, and modulated into radio frequency (RF) signals that are transmitted for reception by WCDs, such as mobile telephones. Upon reception, these signals are converted into baseband signals, despread, deinterleaved, and decoded into the originally transmitted information symbol sequences.
Transmissions between a transmitter and a receiver in a CDMA system are spread by a PN sequence in the transmitter and despread by the same PN sequence in the receiver. In IS-95 systems, these PN sequences have lengths of 215 symbols (“chips”) and have a chip rate of 1.228 MHz. Therefore, the PN sequence period is 26.667 milliseconds. For proper reception of information, PN sequence generation processes in a transmitter must be aligned with PN sequence generation processes in the receiving WCD.
In such systems, interleaving is based on an interleaving time frame, such as 20 milliseconds in IS-95 systems. For proper reception of information in a WCD, both the interleaving process at a transmitter and the deinterleaving process at the receiving WCD must be also synchronized with one another.
In addition, a WCD needs to maintain internal synchronization between its PN sequence despreading process(es) and its deinterleaving process(es). That is, when a WCD transitions from a sleep mode to an awake mode, its PN sequence despreading processes and deinterleaving processes must reinitiate operation as if the WCD was awake during intervening sleep mode intervals. In essence, the WCD must wake up and continue to operate as if it never missed a beat.
One technique of ensuring that such internal synchronization occurs involves employing sleep mode intervals that are equal in time to an integer multiple of both the interleaving time frame and the PN sequence period. According to this approach, when WCD transitions from a sleep mode to an awake mode, its processes resume operation at the same points in the PN sequences and interleaving frames in which they were operating when the preceding transition to sleep mode occurred. In addition, since this transition occurs at an integer multiple of both the interleaving time frame and the PN sequence period, these processes are internally synchronized.
The smallest integer multiple of both the interleaving time frame and the PN sequence period is their least common multiple. In the case of IS-95 systems, this least common multiple equals 80 milliseconds (i.e., three times the 26.667 millisecond PN sequence period and four times the 20 millisecond interleaving time frame). Conventional approaches to sleep and awake mode timing employ this least common multiple approach. Therefore, in IS-95 systems, sleep mode intervals according to this approach are constrained to a coarse granularity of 80 milliseconds.
For a WCD powered by a battery or a fixed energy source, time in a standby state can be increased by reducing the device's average current consumption. One way to achieve this reduction is to increase the duration of the sleep mode interval while reducing the duration of the awake mode interval. Generally, the fraction of time spent in an awake mode is much smaller than the amount of time spent in a sleep mode. However, since current consumed in an awake mode is generally several times greater than the current consumed during a sleep mode, any reduction in the amount of time spent in the awake mode can result in a significant improvement in current consumption and standby time.
Accordingly, what is needed is a technique that reduces the amount of time a WCD spends in an awake mode, thereby increasing the device's operational time.